1. Field of the Invention
This invention relates to a present position detection system, especially to a vehicle present position detection apparatus that performs a dead-reckoning navigation calculation using a travel distance of a vehicle calculated based on a travel direction of the vehicle and an output from a vehicle speed sensor wherein an accuracy of position detection can be improved even if a vehicle speed pulse is missed, and a vehicle present position display apparatus, a navigation system using the position detection, and a recording medium that stores a program for realizing a present position calculation.
2. Related Art
There has been known a navigation system, which displays a present position moving as a vehicle travels on a display unit with a road map, and performs a route guidance by setting an adequate route from the present position to a destination. Such a navigation system contributes smoother driving.
In the case of displaying the vehicle present position and performing the route guidance, it is essential to detect the vehicle present position. A conventional position detection is shown in Japanese patent application Laid-open No. Hei. 8-54248. This position detection is realized by a dead-reckoning navigation calculation using both an azimuth change amount calculated based on an output from a gyroscope and a travel distance calculated based on an output from a vehicle speed sensor. However, since this technique is a "self-contained navigation" in which an own position is detected by itself, an absolute position can not be detected. Therefore, some countermeasures, in which the absolute position is detected by using radio for radio navigation sent from, for example, the GPS (Global Positioning System) satellite or road side beacon, have proposed to adequately detect the present position.
Here, in the case of using the radio from the GPS satellite, it needs to assume a generation of error of approximately 100 meters. Therefore, it is desired to make a correction using a so-called map-matching to improve an accuracy of the position detection. The map-matching makes a position estimation by comparing a travel locus of the vehicle until the present position calculated based on the dead-reckoning navigation (the self-contained navigation only, or adding the radio navigation) with road data based on map data.
However, when the error of the travel distance using the dead-reckoning navigation calculation becomes large, the present position may be matched to the wrong road as a result of a correction by the map-matching. It is thought that the wrong matching is due to the accuracy of pulse data from the vehicle speed sensor.
The vehicle speed sensor outputs pulse signals at an interval related to a travel speed. When a distance corresponds to one pulse interval is set to, for example, 0.4 meters, it can be detected to travel for 40 meters by counting 100 pulses, and it can detect a vehicle speed by dividing 40 meters by a time to output the 100 pulses. It may not be a problem if a detection ability of the vehicle speed sensor is high, for example, the speed sensor can output the pulse signal even less than 1 km/h. However, generally, an actual speed sensor can not obtain the pulse data when the vehicle speed is less than, for example, 3.2 km/h. This is because the vehicle speed sensor itself can not output the pulse signal at low speed, or the accuracy of pulse signal is lowered when the pulse signal is used as the pulse data after the vehicle speed sensor outputs the pulse signal even at less than 1 km/h.
That is, the accuracy of the pulse data is lowered when, for example, the pulse data from the vehicle speed sensor is commonly used by such as an ABS-ECU (Antilock Brake System Electronic Control Unit) and the other vehicle ECU. When the accuracy of the pulse signals is kept to high, a process load to deal with such a high accuracy pulse signal becomes high and it leads to increase a cost. Furthermore, the ABS-ECU does not need to input the pulse data at less than 1 km/h. These are the reasons the accuracy of the pulse data is lowered.
When the accuracy is lowered in that way, errors are accumulated during multiplying the travel distance for the present position detection. For example, when the vehicle repeats starting and stopping due to a traffic signal, a traffic congestion or the like, a vehicle speed pulse missing (hereinafter; pulse-missing), in which the pulse signal can not be obtained due to low speed travel less than, for example, 3.2 km/h, may occur. Although the travel distance due to the pulse-missing per one stopping and starting is approximately some meters at most, the travel distance will be more than some tens meters when the pair of stopping and starting is repeated by 10 times. Therefore, the travel distance may be detected as short as some tens meters than actual travel distance when the travel distance during the pulse-missing is accumulated by repeating the stopping and starting duo to the traffic congestion or the like. In that case, when the vehicle travels a road which intersects a plurality of roads at a short distance interval like a matrix, and when the vehicle turns at one of intersection, the vehicle may be detected to turn at an adjacent intersection before the actual intersection as a result of wrong map-matching due to the pulse-missing.
This inconvenience may be solved if an additional distance sensor for navigation ECU is provided, however, a number of sensors and harnesses of the sensors may increase in the vehicle as a whole. As a basic concept of vehicles, a number of the sensors and the harnesses is prevented from increasing by connecting a plurality of ECUs with a vehicle LAN (Local Area Network) and sharing data from the specific sensor among the ECUs in order to prevent the structure from complicating and to prevent a cost from increasing. Therefore, it is desired to improve the accuracy of a travel distance calculation and the accuracy of present position detection even if the accuracy of the pulse data from the vehicle speed sensor is relatively low.